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Although optional parameters are actually a very good feature, C# does not support them. Now what about assemblies created in VB.Net with methods having optional parameter, how to call them from C# code? Since C# doesn’t support the concept of optional parameter, if you don’t supply required parameters while making a call to such methods, compiler will throw an error.

So how to handle this? There are many ways to address this issue:
1. If the required parameter is of any reference type such as String then you can use
System.Reflection.Missing.Value or System.Text.Missing
While making call to such methods pass the above value as optional parameter.
2. Create a Wrapper class in VB.Net and overload the method with different available options. This assembly could be referenced by the original C# application.

Now the upcoming version of C# 4.0 is going to have a feature of Optional Parameter
C# 4.0 can both declare and consume optional parameters. Here’s a sample of a very simple method that declares a parameter as optional: